1. Field of the Invention
This invention relates broadly to sonic sensors. More particularly, this invention relates to an acoustic sensor used in downhole tools.
2. State of the Art
Knowledge of a diameter of a borehole while it is being drilled is important to the driller because remedial action may be taken by the driller in real time, preventing the delay inherent in tripping the drill string and conducting open-hole logging activities. If, for example, the diameter of the borehole is over-gauge, such a fact may indicate that there is inappropriate mud flow, or an improper mud chemical characteristic, or that the well hydrostatic pressure is too low, or that there is some other source of wellbore instability. If, on the other hand the diameter of the borehole is below gauge or nominal size, such a fact may indicate that the bit is worn and should be replaced so as to obviate the need for later well reaming activities.
Well bore diameter instability increases the risk that the drilling string may become stuck downhole. A stuck drill string implies an expensive and time consuming fishing job to recover the string or deviation of the hole after the loss of the bottom of the drilling string. Well bore diameter variation information is important to the driller in real time so that remedial action may be taken.
Well bore diameter as a function of depth is also important information for a driller where the borehole must be kept open for an extended portion of time. Monitoring of well bore diameters when the drill string is tripped out of the borehole provides information to the driller regarding proper drilling fluid characteristics as they relate to formation properties.
Knowledge of borehole diameter also aids the driller when deviated holes are being drilled. When a borehole is out of gauge, directional drilling is difficult because the drill-string, bottom-hole assembly, and collar stabilizers do not contact the borehole walls as predicted by the driller. Real time knowledge of borehole diameter provides information on which to base directional drilling decisions. Such decisions may eliminate the need for tripping the string so as to modify the bottom-hole assembly to correct a hole curvature deviation problem.
Real time knowledge of well bore diameter is similarly important in logging while drilling (LWD) operations. Certain measurements, especially nuclear measurements of the formation, are sensitive to borehole diameter. Knowledge of the well bore diameter under certain circumstances can be critical for validating or correcting such measurements.
Because real-time knowledge of the borehole diameter is desirable, ultrasonic sensor assemblies for measuring the diameter of a well while it is being drilled are known. For example, U.S. Pat. No. 5,130,950, which is hereby incorporated by reference in its entirety, discloses such a sensor assembly. As seen in prior art FIG. 1 hereof, the prior art sensor assembly 10 is adapted for placement in the wall 12 or stabilizer fin of a drilling collar, which is placed above the drilling bit of a downhole drilling assembly. The ultrasonic sensor assembly 10 includes a sensor stack 14 including an inner sound absorbing backing element 16, a piezoelectric ceramic disk transducer 18 stacked outwardly adjacent the backing element 16, and a matching layer 20 disposed outwardly adjacent the transducer 18 and operating to match the impedance between the ceramic transducer 18 and the environment outside the sensor assembly; i.e., the borehole mud. A window 24, made of PEEK (polyetheretherketone), available from Victrex USA. Inc. of West Chester, Pa., is disposed outwardly of the matching layer 20 of the stack 14, and includes an outwardly facing depression 26 for focusing an ultrasonic pulse into the drilling mud 28 toward the borehole wall 30. An elastomer or epoxy 32 fills the depression to present a smooth face to the flowing mud and the borehole wall.
The backing material 16, the transducer 18, and the matching layer 20 are all held together by glue 22 and provided in a protective rubber jacket 34. The glue 22 prevents the layers of the stack from moving side-to-side relative to each other within the rubber jacket, as the layers must remain in proper alignment for satisfactory performance. An elastomeric material 36 is placed between the rubber jacket 34 and a metallic cup 38 in which the sensor stack 14 is placed. The window 24 is mounted on a spring 40 in the cup 38 outwardly of the rubber jacket 34 and the elastomeric material 36, which surround the sensor stack 14.
The ceramic disk transducer 18 is approximately 0.80 inches in diameter and 0.07 inch in width, and is limited in size by the space required for the rubber jacket 34, which surrounds and seals the sensor stack 14 within the cup 38.
The sensor assembly 10 also includes electrodes 42 attached to the outer and inner surfaces of the ceramic disk transducer 18 and connector pins 44 for connecting the assembly to an electronics module (not shown) disposed within the drilling collar. The electronics module includes control and processing circuitry and stored logic for emitting ultrasonic pulses in the range of approximately 600 KHz to 700 KHz via the ceramic disk transducer 18 and for generating return (also called echo) signals representative of echoes which return to the transducer 18 after bouncing off the borehole wall. The electronic module also preferably includes a source of electrical energy and downhole memory for storing signals as a function of time. It interfaces with an MWD telemetry module for transmitting measurement information to the surface in real time while drilling.
Two sources of noise are present in the vicinity of the sensor stack of the tool. The first can be characterized as drilling noise, which is of a lower frequency band than that of the acoustic pulse-echo of the transducer. The second is pumping noise, which is characterized by a frequency band, which extends into the frequency range of the pulse-echo apparatus.
Pumping noise is mechanically filtered not only by the rubber jacket 34 surrounding the sensor stack 14, but also by a filter ring 46 mounted radially outwardly of the transducer 18 about the rubber jacket. The backing element 20 is shock protected by rubber packing between it and the elastomeric material, which envelops the stack.
Drilling noise, which may be of extremely high amplitude, is partially mechanically filtered by the rubber jacket 34 and filter ring 46 described above and partially electronically filtered. Electronic filtering is achieved by an electronic high-pass filter placed prior to signal amplification to avoid amplifier saturation, which could mask ultrasonic signal detection during amplifier saturation and recovery time.
Even in view of the above, it has been found that the assembly of the ultrasonic sensor system of the U.S. Pat. No. 5,130,950 is not particularly effective in signal transmission and recovery. One reason for this is that the glue between each of the layers of the stack imparts undesirable attenuation to the recovered signal. Another reason is that there is insufficient signal output from the sensor disk, which is limited in size by the rubber boot. In addition, the prior art sensor system is difficult to manufacture, and requires a relatively large amount of time to manufacture as the glue and the elastomeric material must cure between manufacturing steps. Moreover, there is inconsistency between the performance of different sensor systems due to different amount of glue that is manually placed between the layers of the stack.
It is therefore an object of the invention to provide an ultrasonic sensor capable of determining the tool standoff from the borehole wall, and hence the well diameter section at a given depth.
It is another object of the invention to provide an ultrasonic sensor assembly that is optimized for signal recovery.
It is also object of the invention to provide an ultrasonic sensor assembly which positions a sonic sensor relatively closer to the formation, and which enables the use of a relatively larger backing volume.
It is a further object of the invention to provide an ultrasonic sensor that does not utilize glue between the layers of the stack.
It is an additional object of the invention to provide an ultrasonic sensor that is adapted for operation under high pressures.
It is yet another object of the invention to provide an ultrasonic sensor that can be easily manufactured to consistent specifications.
It is yet a further object of the invention to provide an ultrasonic sensor that is relatively inexpensive to manufacture.
In accord with these objects, which will be discussed in detail below, an ultrasonic sensor assembly for measuring the diameter of a well while it is being drilled is provided. The sensor assembly includes a sensor stack including an inner sound absorbing backing element, a piezoelectric ceramic disk sensor stacked outwardly adjacent the backing element, and a matching layer, preferably made of PEEK and disposed outwardly adjacent the ceramic disk. The layers of the stack are provided within a cavity of a metal cup, without a surrounding rubber boot, and are maintained in position by a flexible diaphragm preferably made of PEEK. The diaphragm is sealed with an O-ring. A window, also preferably made of PEEK, is provided against the diaphragm and is held against the diaphragm by a spring.
The diaphragm and O-ring of the invention form a hydraulic seal between fluid/mud in the well and the sensor stack, and operate to retain the layers of the sensor stack. In addition, the diaphragm is acoustically coupled to the sensor stack, thereby improving signal recovery relative to prior art ultrasonic sensor assemblies that utilize glue to hold the layers of the stack together. In addition, the diaphragm deforms to permit the sensor stack to move when transmitting or receiving, yet is adapted to maintain an acoustic coupling between the sensor stack and the window. Furthermore, the diaphragm permits pressure, temperature and volume compensation of the sensor stack. That is, since the sensor stack is comprised of materials of the different thermal properties, it is expected that the stack will experience some volumetric shrinking due to pressure in conjunction with some volumetric expansion due to temperature effects. The diaphragm accommodates to the changes while maintaining perfect acoustic coupling of all elements.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.